Zonalon

CLINICAL PHARMACOLOGY

Although doxepin HCl does have H1 and H2 histaminereceptor blocking actions,
the exact mechanism by which doxepin exerts its antipruritic effect is unknown.
Zonalon (doxepin) Cream can produce drowsiness in significant numbers of patients, and
this sedation may reduce awareness, including awareness of pruritic symptoms.
In 19 pruritic eczema patients treated with Zonalon (doxepin) Cream, plasma doxepin concentrations
ranged from nondetectable to 47 ng/mL from percutaneousabsorption. Plasma levels
from topical application of Zonalon (doxepin) Cream can result in CNS and other systemic
side effects. Once absorbed into the systemic circulation, doxepin undergoes
hepaticmetabolism that results in conversion to pharmacologically-active desmethyldoxepin.
Further glucuronidation results in urinary excretion of the parent drug and
its metabolites. Desmethyldoxepin has a half-life that ranges from 28 to 52
hours and is not affected by multiple dosing. Plasma levels of both doxepin
and desmethyldoxepin are highly variable and are poorly correlated with dosage.
Wide distribution occurs in body tissues including lungs, heart, brain, and
liver. Renal disease, genetic factors, age, and other medications affect the
metabolism and subsequent elimination of doxepin. (See PRECAUTIONS -
DRUG INTERACTIONS.)

Last reviewed on RxList: 12/29/2008
This monograph has been modified to include the generic and brand name in many instances.